1. Field of the Invention
The present invention relates to a very high frequency television tuner circuit. More specifically, the present invention relates to an improvement in a circuit configuration at the radio frequency amplifying stage in a VHF television tuner for selection of VHF television channel signals and a UHF/VHF converted television channel signal through mechanical switching selection of inductance coils.
2. Description of the Prior Art
Typically, a very high frequency (or VHF) television tuner includes a mechanical switching selection scheme of inductance coil means for selection of a broadcasting television signal, a radio frequency amplifier operatively coupled to said inductance coil means for selectively amplifying a desired radio frequency signal, a local oscillator operatively coupled to said inductance coil means for providing a local oscillation frequency signal, the frequency of which is different by a given frequency difference from said selectively amplified radio frequency signal, a mixer responsive to the selectively amplified radio frequency signal from the radio frequency amplifier and the local oscillation signal from the local oscillator for providing an intermediate frequency signal, and an intermediate frequency tuned circuit for selectively withdrawing the said intermediate frequency signal.
The above described mechanical switching selection scheme of inductance coil means may comprise a so-called turret type structure, i.e., a plurality of sets of coils, each set allotted for one channel, provided on a turret rotatably mounted so as to be rotated by means of a channel selection shaft, and a switching contact arrangement for selection of a desired set of coils as a function of manual rotation of the channel selection shaft. Alternatively, the above described mechanical switching selection scheme of inductance coil means may comprise a so-called switch type structure, i.e., a plurality of sets of coils, each set allotted for one channel, provided in a stationary manner along the periphery around a given axis, and a switching contact arrangement rotatably mounted about the axis so as to be rotated by means of a channel selection shaft.
FIG. 1 is a block diagram of a typical VHF television tuner VT. The tuner VT is shown connected to an antenna VA for receiving a broadcasting VHF television wave and comprises a radio frequency amplifier RFA preceded and succeeded by a preceding stage radio frequency tuned circuit RFT1 and a succeeding stage radio frequency tuned circuit RFT2, respectively, for selectively amplifying the desired radio frequency signal, a local oscillator OSC for providing an oscillation frequency signal, the frequency of which is different by a given frequency difference from said selected or desired radio frequency signal, a mixer MIX responsive to said received radio frequency signal from the succeeding stage radio frequency tuned circuit RFT2 and to said local oscillation frequency signal from said local oscillator OSC for providing an intermediate frequency signal, and an intermediate frequency tuned circuit IFT for selectively withdrawing said intermediate frequency signal, and which is shown succeeded by an intermediate frequency amplifier IFA in a television receiver.
The tuner shown in FIG. 1 is also shown connected to a UHF/VHF converter UT which is connected to an antenna UA for receiving a broadcasting UHF television wave. Typically the UHF/VHF tuner is structured to convert a UHF television signal into a VHF television signal of a predetermined frequency and having the same television information. Accordingly, the VHF television tuner VT is adapted to receive selectively the above described VHF television signal of a predetermined frequency.
The high frequency tuned circuits RFT1 and RFT2 each comprise inductance coils L1 and L2, respectively, which are to be selected for each channel by means of a mechanical switching selection means (not shown in FIG. 1) described previously, and each are shunted by capacitors C1 and C2, respectively, to form radio frequency tuned circuits. The oscillator OSC also comprises an inductance coil L3, which is also selected for each channel by means of the mechanical switching selection means (not shown in FIG. 1) and is shunted by a capacitor C3 to form an oscillator tuned circuit OSCT. Although the inductance values of the coils L1, L2 and L3 are changeable by means of the mechanical switching selection means, and hence the frequencies of the tuned circuits RFT1, RFT2 and OSCT are accordingly changeable for the purpose of selection of a desired channel, the difference between the frequencies of the radio frequency tuned circuits RFT1 and RFT2 and the oscillator tuned circuit OSCT is kept constant, so that a constant intermediate frequency signal may be obtained from the mixer MIX. Therefore, the intermediate frequency tuned circuit IFT is tuned to the predetermined intermediate frequency.
In implementing a VHF television tuner as shown in FIG. 1 for selection of VHF television channel signals and a UHF/VHF converted television channel signal through mechanical switching selection of inductance coils, it is necessary to selectively connect a few capacitors to the selected radio frequency inductance coils simultaneously with switching selection of the radio frequency inductance coils, as is well known to those skilled in the art. In addition, the tuner should be capable of switching the input terminal to selectively receive a VHF television signal or a UHF/VHF converted television signal. Hence, a conventional VHF television tuner for selection of VHF television channel signals and a UHF/VHF television signal through mechanical switching selection of inductance coils comprises a complicated mechanical switching scheme.
FIG. 2 shows a schematic diagram of a conventional VHF television tuner circuit for selection of a VHF television signal and a UHF/VHF converted television signal through mechanical switching selection of inductance coils, in which only the preceding radio frequency tuned circuit RFT1 and a radio frequency amplifier RFA are schematically shown. A VHF signal input terminal VIP is connected through an antenna filter AF to the radio frequency tuned circuit RFT1. The antenna filter AF comprises a well known high pass filter of an LC type and is provided to prevent any interference signal of the intermediate frequency band from being transferred to the subsequent stages. The output from the antenna filter is connected to the junction of capacitors C11 and C12 which constitute a capacitance divider circuit and serve to contribute a portion of the capacitance for the radio frequency tuned circuit RFT1.
The mechanical switching scheme of the embodiment shown comprises four sets of switches to implement the radio frequency tuned circuit RFT1. More specifically, the mechanical switching scheme for the radio frequency tuned circuit RFT1 comprises four blades or movable contacts 21, 22, 23 and 24, each operatively associated with stationary contacts, as shown in the figure. The first and second blades 21 and 22 are formed on the front and rear surfaces of a rotor, not shown, fixedly provided on the channel selection shaft at the corresponding sections in the tuner. Similarly, the third and fourth blades 23 and 24 are also formed on the front and rear surfaces of another rotor, not shown, fixedly provided to the channel selection shaft at the respective sections in the tuner. These blades 23 and 24 are also electrically interconnected. The first blade 21 is adapted to short circuit a capacitor C1 if and when the first set of switches implemented by the blade 21 is turned to any one of low band channels and to open circuit the same for the remaining channels. To that, two stationary contacts connected across the capacitor C1 are adapted to be in contact with the largest diameter portion of the blade 21 corresponding to the low band channels so as to short circuit the capacitor C1 only for these low band channels. On the other hand, the second blade 22 is aimed to short circuit another capacitor C2 only for the high band channels and the UHF/VHF converted channel. According to the Japanese television standard system, the low band channels include channel Nos. 1, 2 and 3 and the high band channels include channel Nos. 4 through 12. This blade 22 is also aimed to selectively switch the inductance coils. To that end, coil elements, spirally wouned, are each connected between the adjacent two stationary contacts 1 through 12. For the purpose of individual selection of inductance coils for each channel, the blade has the largest diameter portion of one channel width so as to individually contact each stationary contact. In order to short circuit the capacitor C2 only for the high band channels, the blade 22 further has a medium diameter portion to cover the high band channels, while the remaining portion of the blade 22 is selected to be the smallest diameter portion. The stationary contacts 1 and u are connected across the capacitor C2 and the contact 1 is extended to reach the smallest diameter portion while the stationary contact u is extended to reach the medium diameter portion of the blade 22. The third blade 23 is allotted for the purpose of selectively short circuitting another capacitor C3 to be selectively inserted between the radio frequency tuned circuit RFT1 and the radio frequency amplifier RFA. To that end, the blade 23 has the high band channel portion formed of a larger diameter and the low band channel portion formed of a smaller diameter. The stationary contact is selected to reach the larger diameter. The fourth blade 24 is used to selectively connect a UHF/VHF converted signal input terminal to the radio frequency amplifier RFA. To that end, the UHF/VHF converted input signal terminal UIP is connected to a VHF tuned circuit VFT and the output therefrom is connected through a capacitor C5 to a stationary contact corresponding to the UHF/VHF converted channel position. The fourth blade 24 has the VHF/UHF converted channel portion formed of a larger diameter and the remaining portion formed of a smaller diameter so that, if and when the channel switching scheme is turned to the UHF/VHF converted channel, the UHF/VHF converted signal input terminal UIP is coupled to the radio frequency amplifier RFA.
FIG. 3A shows an equivalent circuit of the FIG. 2 diagram when the channel selection switch is turned to any one of high band channels, FIG. 3B shows an equivalent circuit displaying in a simplified manner the FIG. 3A equivalent circuit, FIG. 3C is an equivalent circuit of the FIG. 2 diagram when the channel selection switch is turned to any one of the low band channels, and FIG. 3D shows an equivalent circuit of the FIG. 2 diagram when the channel selection switch is turned to the UHF/VHF converted channel. It is pointed out that, referring to FIGS. 3A and 3B, since the capacitance value of the capacitor C4 is relatively large in terms of the high band channel frequency, the capacitance C4 may be considered as an inductance.
As seen from the foregoing description with reference to FIGS. 2 and 3, it is seen that a conventional VHF television tuner for selection of VHF television channels and a UHF/VHF converted channel through mechanical switching selection of inductance coils uses one blade for selective connection of each capacitor to be selectively connected to the selected inductance coil between the high and low band channels, and also uses one separate blade for selection of the UHF/VHF converted channel. However, this increases the number of sections and the number of blades in the channel selection switch arrangement. This accordingly increases the number of stationary contacts or clips. Thus, the structure of the channel selection switch arrangement for use in the above described VHF television tuner becomes complicated, which makes the cost expensive. In addition, more time is consumed for assemblage of such a VHF television tuner.